Differential Scanning Calorimetry
| Linseis Chip DSC 10 |
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| Maximum Temperature: 500C |
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 (), melting points, cold crystallization temperature (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T_c } ), and recrystallization temperature (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 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 Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle T_g } ). The enthalpy values of cold crystallization (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta H_c } ), melting (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta H_m } ), and recrystallization (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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 (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle C_p } ) at a certain temperature can be calculated by dividing the heat flow at that temperature by the heating rate. The percentage crystallinity (Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle X_C } ) was calculated from the following equation:
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle X_C = \frac{\Delta H_m}{\Delta H^\circ_m} \cdot 100\% }
where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta H^\circ_m } 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.