Differential Scanning Calorimetry

Linseis Chip DSC 10
Maximum Temperature: 500$^{\circ }$C

The Linseis Differential Scanning Calorimeter (DSC) is a thermal testing instrument used for measurements of material properties such as transformation temperatures, enthalpies of transformation, and heat capacities. It is capable of measuring temperatures up to 500 degrees Celsius and can use both solid and liquid specimens.

Applications

The glass transition temperatures ($T_{g}$), melting points, cold crystallization temperature ($T_{c}$), and recrystallization temperature ($T_{rc}$) can be found from the peaks in the DSC plot or local maximum of the first derivative of heat flow vs. temperature (for $T_{g}$). The enthalpy values of cold crystallization ($\Delta H_{c}$), melting ($\Delta H_{m}$), and recrystallization ($\Delta H_{rc}$) transitions (where applicable) for each sample can be determined from trapezoidal Riemann sums which calculate the area under each peak. The heat capacity of each sample ($C_{p}$) at a certain temperature can be calculated by dividing the heat flow at that temperature by the heating rate. The percentage crystallinity ($X_{C}$) was calculated from the following equation:

$X_{C}={\frac {\Delta H_{m}}{\Delta H_{m}^{\circ }}}\cdot 100\%$                      

where $\Delta H_{m}^{\circ }$ is the melting enthalpy for a 100% crystalline sample (found in literature).

Test Parameters

DSC has several test parameters that are crucial to set to proper values based on the sample(s) being tested in order to obtain the best data possible. These include the endpoints, ramp rate, and number of heating/cooling cycles. The endpoints when testing an unknown polymer should be determined by the maximum temperature that the DSC model is capable of and the degradation temperature of that polymer (can be found from TGA). Changing the ramp rate of the DSC can affect the sensitivity and resolution of the measurements and the appearance of the curves. A high ramp rate (such as 50°C/min) increases sensitivity at the cost of resolution, while a very low ramp rate (0.1°C/min) could cause the sample to undergo unwanted annealing/crystallization. Multiple heating/cooling curves are used when a polymer has undergone processing to find its intrinsic values, while the first heating curve shows any processing the polymer has undergone.

In a situation where the initial heating curve of a semi-crystalline polymer such as PET shows a certain Tg, Tc, and Tm, the cooling rate can have a significant effect on the values of these parameters found in the following heating cycle. For a slow cool (~1°C/min) these values would be expected to decrease, while they would be expected to increase due to a very fast cool (~1000°C/min). The enthalpy of cold crystallization would be expected to decrease while enthalpy of melting would be expected to increase in the former scenario and while the opposite would be expected in the latter scenario.

Cold Crystallization vs. Recrystallization

While similar sounding, cold crystallization and recrystallization are different processes and are easily discernible in DSC analysis. Cold crystallization is an exothermic process that occurs during heating and consists of previously amorphous regions rearranging themselves into new crystallites from the thermal energy being put into the polymer during heating. An example of when cold crystallization occurs is when a semi-crystalline polymer has been rapidly quenched during processing, so it has been frozen in an amorphous state. Re-crystallization is also exothermic, but occurs during cooling and arranges molten polymer chains into ordered crystalline structures.

meas