DSC Heat/Cool/Heat experiments are designed to erase previous thermal history by heating the material above a transition (e.g., glass transition or melting), where relaxation or molecular rearrangement can occur, then cooling at a known rate before heating again. The first heating curve provides the “as received” information. The cooling imparts a known thermal history. Therefore, any differences observed between similar materials in the second heating curve are related to real internal differences in the materials (e.g., molecular weight) rather than previous thermal history effects.
If you want to set up a DSC heat/cool/heat procedure, you can choose one of two techniques—the DSC Wizard or the Experimental View - Procedure Page. See the instructions below for each technique.
When you perform a DSC Heat/Cool/Heat experiment, the following parameters need to be chosen:
Start Temperature: The actual starting and upper temperatures chosen for a specific experiment must be within the usable temperature of the DSC cell/cooling accessory combination that you are using. The RCS and LNCS can be used for heat/cool/heat experiments spanning subambient to elevated temperatures. The FACS can be used for heat/cool/heat experiments from 50 to 500°C. The Quench Cooler cannot be used for heat/cool/heat experiments. These temperatures should bracket the region where sample transitions are expected to occur and they should be far enough below or above the transitions to establish a stable baseline.
You can choose to start your experiment from one of the following choices:
Current Temperature—the temperature of the DSC cell at the current moment (usually ambient temperature, 35°C). In this case, the ramped heating begins immediately on starting the experiment.
Another Temperature—you choose the specific temperature desired (default is 50°C). In this case, the system equilibrates at the chosen temperature before ramped heating begins.
Upper Temperature: To obtain the intended benefits from a heat/cool/heat experiment, the upper temperature chosen should be higher than the transition that "erases" previous thermal history. This is typically 10 to 15°C above the glass transition or melting peak. However, the upper temperature should also be low enough to prevent the onset of decomposition.
Lower Temperature: The lowest temperature achieved during the cooling portion of the experiment should be below any transitions of interest.
Heating Rate & Cooling Rate: The acceptable range of heating/cooling rates for conventional DSC is 0.01 to 100°C/minute. This range depends on a number of variables including the temperature range covered and the presence of a cooling accessory. Faster heating/cooling rates generally increase sensitivity particularly for thermal events such as the glass transition. They also obviously shorten the time of analysis. Slower heating/cooling rates generally provide better resolution (separation of closely spaced thermal events). Heating rates of 10 to 20°C/minute and cooling rates of 5 to 10°C/minute are a good starting point for most materials.
Additional Conditions: There are additional experimental conditions that can be changed to optimize specific situations. These are accessed using Advanced Parameters and Post-Test Conditions. For most experiments, the system default values shown are recommended.
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Enter the desired parameters, then select Next to proceed.
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Display the Procedure Page. Select the desired experiment from the list of available preprogrammed templates.
Enter the desired parameters. When you have finished setting up this procedure, you can enter more information by selecting the Notes Page and Summary Page for this run.
Click the Apply button when finished to save these changes or the Append button to save the changes to this run and add another run to the end of the sequence.
NOTE: You can use any test template as the basis for a custom test by selecting the desired test template from the list of tests, then click the Apply button and change the test to Custom. Then you can click on the Procedure Page and then the Editor button to make the changes desired.
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