Differential scanning calorimetry , or DSC , is a thermoanalytical technique in which the difference in the amount of heat required to increase sample temperature and reference is measured as a function of temperature. Both the sample and the reference were maintained at nearly the same temperature throughout the experiment. Generally, the temperature program for DSC analysis is designed so that the temperature of the sample holder increases linearly as a function of time. The reference sample must have a well-defined heat capacity over the temperature range to be scanned.
This technique was developed by E. S. Watson and M. J. O'Neill in 1962, and was introduced commercially at the Pittsburgh Conference of 1963 on Analytical Chemistry and Applied Spectroscopy. The first adiabatic differential scanning calorimeter that can be used in biochemistry was developed by P. L. Privalov and D. R. Monaselidze in 1964 at the Institute of Physics in Tbilisi, Georgia. The term DSC was created to describe this instrument, which measures energy directly and enables precise heat capacity measurement.
DSC Type:
- power-compensated DSC , maintaining a constant power supply
- DSC Heat-flux , making the heat flux constant
Video Differential scanning calorimetry
Deteksi transisi fase
The basic principle underlying this technique is that when the sample undergoes a physical transformation such as phase transition, the required heat will flow less than the reference to keep both at the same temperature. Whether less or more heat should flow into the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts into a liquid, it will require more heat flowing into the sample to raise the temperature to the same level as the reference. This is due to heat absorption by the sample due to endothermic phase transition from solid to liquid. Likewise, since the sample undergoes an exothermic process (such as crystallization) less heat is required to raise the sample temperature. By observing differences in heat flow between samples and references, the differential scanning calorimeter can measure the amount of heat absorbed or released during the transition. DSC can also be used to observe subtle physical changes, such as glass transitions. It is widely used in industrial settings as a quality control instrument because of its application in evaluating the purity of the sample and for studying polymer treatment.
Maps Differential scanning calorimetry
DTA
An alternative technique, which has much in common with DSC, is differential thermal analysis (DTA). In this technique is the heat flow to the sample and the reference that remains the same than the temperature. When samples and references are heated identically, phase changes and other thermal processes cause the temperature difference between the sample and the reference. Both DSC and DTA provide similar information. DSC measures the energy required to store references and samples at the same temperature while the DTA measures the temperature difference between samples and references when the same amount of energy has been put into both.
DSC curve
di mana adalah entalpi transisi, adalah konstanta kalorimetri, dan adalah area di bawah kurva. Konstanta kalorimetri akan bervariasi dari instrumen ke instrumen, dan dapat ditentukan dengan menganalisis sampel yang dikarakterisasi dengan baik dengan entalpi transisi yang diketahui.
Aplikasi
Differential calorimetry scanning can be used to measure a number of characteristic properties of a sample. Using this technique it is possible to observe fusion events and crystallization as well as the glass transition temperature T g . DSC can also be used to study oxidation, as well as other chemical reactions.
Glass transitions may occur as the temperature of the amorphous solid increases. This transition appears as a step in the baseline of the recorded DSC signal. This is because the sample changes in heat capacity; no formal phase changes occurred.
As the temperature increases, the amorphous solid becomes less viscous. At some point, the molecule can obtain enough freedom of movement to spontaneously arrange itself into a crystalline form. This is known as the crystallization temperature ( T c ). The transition from an amorphous solid to a crystalline solid is an exothermic process, and produces a peak in the DSC signal. As the temperature increases, the sample finally reaches its melting temperature ( T m ). The melting process produces an endothermic peak on the DSC curve. The ability to determine transition and enthalpy temperatures makes DSC a valuable tool in generating phase diagrams for various chemical systems.
Example
This technique is widely used in various applications, both as a routine quality test and as a research tool. The equipment is easily calibrated, using low melting indium at 156,5985 Â ° C for example, and is a fast and reliable thermal analysis method.
Polymers
DSCs are widely used to inspect polymeric materials to determine their thermal transitions. The observed thermal transitions can be used to compare the material, although the transition does not uniquely identify the composition. Unknown material compositions can be solved using complementary techniques such as IR spectroscopy. The melting point and glass transition temperature for most of the polymers are available from standard compilations, and this method can show polymer degradation by lowering the expected melting point, T m , for example. <<> <<> sub depends on the molecular weight of the polymer and the thermal history, so the lower values ​​may have lower than expected melting points. The percent crystal compound of the polymer can be estimated from the crystallization/melting of the peak DSC graph as heating the reference fusion can be found in the literature. DSC can also be used to study thermal degradation of polymers using approaches such as Oxidative Onset Temperature/Time (OOT), however, the risk of DSC cell contamination users, which can be a problem. Thermogravimetric analysis (TGA) may be more useful for the determination of decomposition behavior. Dirt in the polymer can be determined by examining the thermogram for anomalous peaks, and plasticizers can be detected at their typical boiling point. In addition, the examination of small events in the first thermal analysis thermal data can be useful because it seems that this "anomaly peak" can actually represent the process or storage of thermal history of materials or the physical aging of the polymer. Comparison of the first and second heat data collected at a consistent heating rate can allow the analyst to learn about the history of polymer processing and material properties.
Liquid crystal
DSC is used in the study of liquid crystals. Because some material forms change from solid to liquid, they go through a third state, which displays the properties of both phases. This anisotropic liquid is known as a liquid crystal or mesomorphous state. Using a DSC, it is possible to observe small energy changes that occur as the transition of matter from solid to liquid crystals and from liquid crystals to isotropic liquids.
Oxidative stability
Using differential scanning calorimetry to study stability against sample oxidation generally requires an airtight sample space. Typically, such tests are performed isothermally (at constant temperature) by changing the sample atmosphere. First, the sample is brought to the desired test temperature under an inert atmosphere, usually nitrogen. Then, oxygen is added to the system. Any oxidation occurring is observed as a deviation in the baseline. Such an analysis can be used to determine the optimal stability and storage conditions for a substance or compound.
Security filtering
DSC makes a reasonable initial safety screening tool. In this mode, the sample will be placed in a non-reactive container (often gold or gold-plated), and which will be able to withstand pressure (typically up to 100 bar). The presence of exothermic events can then be used to assess the stability of a substance to heat. However, due to a combination of relatively poor sensitivity, slower than normal scanning levels (typically 2-3 Ã, Â ° C/min, due to more heavier containers) and unknown activation energy, it is important to subtract about 75-100 Â ° C from the initial start of the observed eksoterm to suggest the maximum temperature for the material. A much more accurate set of data can be obtained from adiabatic calorimeters, but such tests can take 2-3 days from the ambient at an increase level of 3 Ã, Â ° C per half hour.
Drug analysis
DSC is widely used in the pharmaceutical and polymer industries. For polymer chemists, DSC is a useful tool for studying the preservation process, which allows fine tuning of polymer properties. Cross-linking of polymer molecules occurring in the preservation process is exothermic, resulting in a negative peak in the DSC curve which usually appears immediately after the glass transition.
In the pharmaceutical industry it is necessary to have well-characterized drug compounds to determine the processing parameters. For example, if necessary to transmit the drug in an amorphous form, it is desirable to process the drug at temperatures below those which may occur crystallization.
General chemical analysis
Frost depression can be used as a purity analysis tool when analyzed with differential scanning calorimetry. This is possible because the temperature range at which the mixture of the compound melts depends on its relative amount. Consequently, less pure compounds will exhibit an expanded melt peak that starts at a lower temperature than a pure compound.
See also
Template: Led
References
- Source Brydson, J. A., Plastic Materials , Butterworth-Heinemann, Ed 7th (1999).
Source of the article : Wikipedia
0 Comments