Also, why on increasing scan rate, glass transition temperature is not much changing if we take sample in less amount and thermal lag is decreased.
The glass transition is a kinetic phenomenon. Its T increases with increasing the heating rate. The size of the actual change dT is proportional to a change in the heating rate, dHR, but inversely proportional to the activation energy of the process. That is dT ~ dHR / E. Because the glass transition occurs cooperatively (simultaneous breaking of multiple intermolecular bonds) its E is usually quite high, i.e., several times larger than E of typical chemical reactions. As a result, dT typically is quite small.
For examples of the values see:
DOI: 10.1002/macp.200600095
When the amount of materials to be tested by DSC and/or TGA is less than the recommended by the vendors, the results can be not reliable. In addition, scan rate will affect the results of, e.g. glass transition, recrystallization and melt, the scale of difference subject to the scan rate and compound. At last, the reproducibility of the thermal test results will get worse with increased scan rate, also subject to the amount of materials tested. Hence, to develop an validated thermal test method, one has to consider all the variables incl the amount of materials, scan rate, initial condition, sample holder, also subject to the compound and equipment.
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