Introduction
Modulated DSC measurements are used to separate overlapping effects. The sample is subjected not only to linear heating rate, but also to sinusoidal temperature variations. This method leads to separation of the so-called reversing and non-reversing part of the heat flow. The reversing effects are a function of temperature and oscillate with temperature variations. The non-reversing processes are a function of time and are calculated as the difference between the total heat flow and the reversing heat flow.
A modulated measurement contains three parameters to be chosen by the user:
- the underlying heating rate
- the amplitude (in K)
- the period of oscillation (in s)
An appropriate heating rate and a sufficient frequency are necessary to ensure that the effects to be separated contain enough oscillations for an improved separation of the effects. This is a required condition for achieving good separation of the reversing and non-reversing processes. Because it is difficult for a heat-flow DSC to follow fast heating rates along with short oscillations, modulated measurements are usually carried out at heating rates of less than or equal to 5 K/min.
Thanks to the low thermal mass of the furnace, the heat-flow DSC 214 Polyma is able to modulate at heating rates of 10 K/min in combination with short periods and high amplitudes for results that are both quickly achieved and accurate.
Test Conditions
A polystyrene sample was prepared in a Concavus® pan and measured with the DSC 214 Polyma. This polymer was heated to 150°C at 10 K/min. Oscillations with a 20 s period and 1 K amplitude were used as modulation parameters. Only a small amount of the polymer (2.36 mg) was used, in order to ensure a homogeneous temperature distribution within the sample despite the fast oscillations and high amplitude.
Test Results
The total measured heat flow (which conforms to a conventional DSC curve) is displayed in figure 1. The EndothermicA sample transition or a reaction is endothermic if heat is needed for the conversion.endothermic step detected at 102°C (midpoint) is due to the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition of polystyrene. It is overlapped with a RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation peak at 108°C resulting from the release of mechanical tension within the sample. The two effects can only be evaluated if they are separated. This can be achieved using temperature modulation.
Figure 2 shows that the temperature is perfectly controlled during the modulated measurement: the underlying heating rate of 10 K/min as well as the amplitude of 1 K are both maintained without any difficulty.
Separation of the total heat flow into reversing and nonreversing signals is shown in figure 3. The Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition occurs in the reversing part of the heat flow whereas the irreversible RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation peak is a typical non-reversing effect. Both effects can now be correctly evaluated: the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition was detected at 105.1°C (midpoint) and the RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation peak at 105.6°C (peak temperature) with an enthalpy of 1.2 J/g.
Conclusion
Thanks to modulation, only a few minutes are required to accurately evaluate the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition of polystyrene. The DSC 214 Polyma combines the robustness of a heat-flow DSC and the advantages of a fast, well-controlled furnace even allowing for temperature-modulated DSC measurements at high heating rates.