A differential scanning calorimeter (DSC) measures the heat flow applied to a sample and to an inert reference for comparison. Heat flow is adjusted to maintain the sample and the reference at the same temperature. As the temperature of the sample increases, the DSC plots a function of heat flow (energy) vs. sample temperature.
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In General Chemistry, we learned that in going from ice to water, additional energy was needed
at the transition (melting) to complete the transition. If we started at -10 C, with the goal to
heat up to 10 C, we would notice a delay when the temperature reached 0 C.
When a polymer is heated, the function of energy vs. temperature will appear "smooth" until it reaches a temperature where a transition begins.
- For a glass transition temperature, additional energy will be needed; this transition is considered endothermic.
- For crystallization, less energy will be needed; crystallization is considered exothermic.
Differential Scanning Calorimetry can be used to determine glass transitions, cold crystallization, crystallization, phase changes, melting, cure kinetics, and oxidative stability.
In the illustration of a DSC plot (for polyetheretherketone), three discontinuities are observed:
- red point- onset of glass transition temperature
- orange point- onset of crystallization peak
- yellow point- onset of thermal melt
There are two types of DSC:
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Heat flux
- Superior baseline
- Power Compensation (developed in the early 1960s)
Cooling- Several cooling options for temperature control include:
- liquid circulation
- refrigerated liquid circulation
- liquid nitrogen
Materials for sample pans:
- aluminum
- aluminum oxide
- copper
- gold
- graphite
- platinum
- silver
- stainless steel
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The heating rate is important, as different heating rates will produce different graphs.
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Example heating rate: 10°C/min
Baseline- temperature difference between the two empty crucibles (sample holders) is measured as temperature changes at a constant rate over the temperature range of interest.
Sapphire discs are used for calibration.
Calibration may also include tests with high-purity melting standards, such as indium and zinc.
Properties that can be determined by Differential Scanning Calorimetry (DSC) include:
- Tg- glass transition temperature
- Tm- crystalline melt temperature
- ΔHm
- ΔHc
DSC may be used to evaluate sample purity.
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While it can't be quantified, it appears from literature survey that DSC is considered the most important
method for the thermal analysis of polymers.
Other Thermal Analysis Pages:
