In quasi-isothermal MDSC experiments the underlying heating rate is zero. However, by selecting a modulation temperature amplitude and period, the material is still exposed to an instantaneous heating rate.
If you want to set up an MDSC quasi-isothermal procedure that permits the measurement of heat capacity under isothermal conditions, you can choose one of two techniques—the DSC Wizard or the Experimental View - Procedure Page. See the instructions below for each technique.
Three variables are adjusted to obtain the desired complex MDSC temperature profile—the underlying heating rate, modulation period, and temperature amplitude of modulation.
Generally, comparable results are obtained for underlying heating rates between 1 and 5°C/minute. However, for evaluation of time-dependent events such as glass transitions, the results obtained vary slightly with heating or cooling rate. Therefore, to obtain the most thermodynamically correct value for the glass transition temperature as well as the heat capacity change during the transition, the underlying heating rate should be set to zero (isothermal). A measurement of heat capacity can still be made under isothermal conditions based on the instantaneous heating rate generated by the modulation.
Start Temperature: In practice, a starting temperature is chosen, which is 10°C below the transition of interest.
Isothermal Time: A stabilization time (10 minutes) is allotted at each temperature to ensure the equilibrium heat capacity value is obtained.
Modulation Period & Modulation Temperature Amplitude: A typical modulation period (60 seconds) and temperature amplitude (+/-0.5°C) are used to generate the modulation.
Temperature Increment: A series of isothermal steps (1°C increments) is run, which covers the transition and 10°C above it.
Number of Increments: Select the number of temperature increments desired. The default is 20.
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.
Modulated DSC® and MDSC® are registered trademarks of TA Instruments—Waters LLC.
Modulated DSC® and MDSC® are terms which describe proprietary technology invented by Dr. Mike Reading of ICI Paints (Slough UK) and patented by TA Instruments—Waters LLC (U.S. Patent Nos. B1 5,224,775; 5,248,199; 5,335,993; 5,346,306).
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