New modulated DSC measurement technique

The introduction of modulated DSC (MDSC) increased the utility of DSC by adding the ability to separate the sample heat flows resulting from time dependent and time independent processes. The time independent sample heat flow rate is just that which is due to the sample heat capacity. MDSC is based on conventional DSC where the measured signal is the temperature difference between the sample and reference positions of the sensing apparatus. The measured heat flow rates display significant dependence on the period of the modulation, as demonstrated by the well-known decrease of measured heat capacity with decreasing modulation period. To reduce the frequency dependence, MDSC experiments are usually performed using rather long periods, typically 60 s or more, or various calibration or correction techniques may be applied. Long periods require low average heating rates and result in rather lengthy experiments that reduce the productivity of the DSC. A new MDSC heat flow measuring technique has been developed that greatly reduces the frequency dependence of the heat flow measurement. It includes three components: (1) a new DSC cell that has independent sample and reference calorimeters and includes two differential temperature measurements, (2) a new DSC heat flow measurement method that is based upon independent sample and reference heat flow rates and (3) a new MDSC heat flow calculation method that uses the independent sample and reference heat flow rate measurements. MDSC experiments using this new technique display greatly reduced period dependence, allowing modulation periods as short as 20 s to be used. Higher average heating rates may be used and productivity of the DSC is increased as much as threefold or more. (C) 2002 Published by Elsevier Science B.V.