The glass transition (Tg) is a reversible physical transition that occurs when the amorphous portion of a material is heated or cooled in a specific temperature range. Cooling a material below its glass transition temperature it becomes brittle like glass, while heating above the Tg the material becomes leathery and then rubbery. To know the characteristic values of the glass transition like glass transition temperature Tg in K or °C and the delta cp value in J/g°C (= the step height of the curve) is useful for determination of the operating temperature range or the amorphous content of a semi-crystalline material.
Physical properties such as the heat capacity, Cp, the coefficient of thermal expansion (CTE) and the storage modulus (G') change at the glass transition. The figure shows three different curves of polystyrene (PS) obtained from Differential Scanning Calorimetry (DSC), Thermomechanical Analysis (TMA) and Dynamic Mechanical Analysis (DMA). The Cp curve was determined from a DSC heating run at 5 K/min. The PS was shock-cooled beforehand. The CTE was obtained from the second heating run with a TMA, and the modulus (G') was determined with a DMA shear measurement at 10 Hz.
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Differential scanning calorimetry (DSC) is a versatile measurement technique deployed in many analytical laboratories, for both QC and R&D. It measures the heat flow produced in a sample when it is either heated, cooled, or held isothermally at a constant temperature.
DSC is a popular route for determining the glass transition due to its ease of use and short measurement times.
A conventional DSC typically covers measurements of up to 700 °C. However, great results can also be obtained above 700 °C using a METTLER TOLEDO Thermogravimetric Analyzer/ Differential Scanning Calorimeter (TGA/DSC).
In the example of Polyvinylchloride (PVC), the glass transition temperature and its delta cp value is shown to be dependent on the degree of chlorination.
Standards
A standard is used to provide a comparative norm that is both widely applied and recognized. This ensures that test results in production and testing processes applying the same standard are uniformly comparable. Standards can – where available – replace method validation in analysis, which is required for quality assurance, accreditation, or approval. Today, standardization in thermal analysis is carried out by many national and international organizations, such as ISO, ASTM, DIN and CEN.
There are many standards available that regulate glass transition determination. Those standards are ISO 11359-2, ISO 11357-2, EN 6032, ASTM E1356 or ASTM D3418 just to name a few.
Tips and Hints
For the most common technique DSC: The measurement of good quality quantitative glass transition data is a challenging task for DSC. Nevertheless, it is a common and useful technique because of the availability of DSC instruments, simple sample preparation, relatively short measurement time, good accuracy, and easy evaluation using commercial software.
Tips and hints for obtaining good data include:
- Stabilize the instrument before the measurement
- Never use the first measurement after switching on the instrument
- Check the reproducibility using repeated measurements
- Averaging different measurements improves the accuracy
- Place the clean crucibles accurately on the sensor
- If possible, use crucibles with only a small difference in mass, <50 µg
- Recommended: 40 µL crucible; sample mass: 10 to 20 mg for organic materials
- Ensure good thermal contact between the sample and the crucible
Applications
Related Products
Differential Scanning Calorimetry (DSC)
DSC measures the enthalpies associated with transitions and chemical reactions and determines the temperature at which these processes occur.
Thermomechanical Analysis (TMA)
Thermal expansion coefficients are determined accurately and precisely by Thermomechanical Analyzers with nanometer resolution from -150 to 1600 °C.
Dynamic Mechanical Analyzer (DMA)
Determine the force and displacement amplitudes as well as phase shifts with the dynamic mechanical analyzer.
FAQ
Which main factors influence the glass transition?
Curing/crosslinking degree, plasticizers, crystallinity and thermal history influence the glass transition.
What are typical results for glass transition obtained from the DSC heat flow curve?
Glass transition temperature, step height, which corresponds to specific heat capacity cp and width of the glass transition.
Why is knowledge of glass transition temperature important?
- For determination of the maximum use temperature of the material
- For adjusting process parameters if the material has to be processed in a certain state
- For determination of curing degree of thermosetting materials
- For determination if a material is crystalline, amorphous or semi-crystalline
How can glass transition be determined above 700 °C?
Glass transition can be measured in a high temperature range using a TGA/DSC.
What are the most sensitive thermal analysis techniques for determining glass transition?
The sensitivity for glass transition increases in the following order: TGA/DSC, DSC, temperature modulated DSC, TMA, Differential Load TMA (DLTMA), and finally DMA. This is due to the order of magnitude the physical property increases that is measured with the respective technique.