Glass points and glass transition temperatures
Glass points play a special role in thermal analysis, especially for polymers. [3, chap. 2.5.6]. However, the crystallization of amorphous metals and the softening of inorganic glasses are also referred to as glass points. [8, S. 160]. For most polymers glass points lie in a range of -100 °C and 100 °C. This is due to glass formation caused by the cooling of the polymer. The liquid polymer solidifies so quickly that not all of the molecular chains can align themselves into crystals and the material forms amorphous portions instead of orienting itself holistically into a crystal structure. In DSC measurements, the glass point is an endothermically directed step and refers to the transition to the rubbery elastic region when a polymer is heated. It is not a sudden change, but occurs gradually over a range of time. If the volume of a sample is plotted against temperature, it can be seen that this increases in the region of the glass transition, which can be attributed to the increasing mobility of the polymer chains. This is illustrated in the following (Fig. 1).
![Figure 1: Specific volume versus temperature in the range of the TG [1, S. 83]](https://www.linseis.com/wp-content/uploads/2021/10/TG-Specific-volume-vs-temperature-300x200.png)
Figure 1: Specific volume versus temperature in the range of the TG [1, S. 83]
The glass transition temperature (hereafter referred to as TG) is not only dependent on the original material properties of the monomers; polymerization during production and external factors such as UV radiation, storage temperature or mechanical penetration also influence the material and thus the glass point. This makes it indispensable in practice for quality control and assurance. If the material cools down too quickly, for example due to injection molds that have not been preheated, the glass transition becomes more visible and its temperature is shifted because the molecular chains have less time to form crystallization nuclei. The material is less cross-linked, which can have a negative effect on mechanical properties [5] . Accordingly, the height, width and temperature of the glass point also allow conclusions to be drawn about cross-linking and chain length of the molecules in the polymer. In addition, some plastics absorb water, which lowers the temperature of the glass transition. Glass points can also be used to determine mixing ratios or the purity of a polymer substance system. Additives such as plasticizers for better processing also lower the glass point of a polymer. In copolymers, the glass point is also an indication of the homogeneous mixing of the monomers. Depending on the materials used, the mixture of substances may have a common glass point or two glass points. To determine a glass transition temperature, three calculation methods have become established. The first is the mid-slope method and is illustrated in Fig. 2. Here, two tangents are placed in the measurement signal immediately before and after the glass transition, the arithmetic mean of these two values is formed and plotted on the DSC signal. A source of error here can be an unclean baseline, since the arithmetic mean shifts due to changes in the rise or incorrect tangent selection due to noise and other interference effects.
![Figure 2: Evaluation of a glass point using the tangent method. [5, p. 58]](https://www.linseis.com/wp-content/uploads/2021/10/Evaluation-of-a-glass-point-using-the-tangent-method-300x110.png)
Figure 2: Evaluation of a glass point using the tangent method. [5, p. 58]
A second method is the inflection point of the glass transition. In the area of the glass transition, the DSC signal is considered as a mathematical function, the inflection point of this function is determined and then output as a glass point. Fig. 3 illustrates this method graphically.
![Figure 3: Evaluation of a glass point using the inflection point method [5, p. 58].](https://www.linseis.com/wp-content/uploads/2021/10/Evaluation-of-a-glass-point-using-the-inflection-point-method-300x131.png)
Figure 3: Evaluation of a glass point using the inflection point method [5, p. 58].
However, due to superposition with a relaxation peak or a ripple in the area of the glass transition, errors can also occur here.
A third calculation method is the Richardson glass temperature determination. Here, as in the tangent method, two straight lines are placed in the baseline before and after the glass transition. For evaluation, the area between these extrapolated straight lines is integrated and thus the fictitious glass transition temperature is determined. The method is described in DIN 51007:2019-4 [9] and is illustrated in Fig. 4.
![Figure 4: Evaluation of the glass point according to Richardson [10, p. 4].](https://www.linseis.com/wp-content/uploads/2021/10/Evaluation-of-the-glass-point-according-to-Richardson-300x200.png)
Figure 4: Evaluation of the glass point according to Richardson [10, p. 4].
[1] N. Jadhav, V. Gaikwad, K. Nair und H. Kadam, „Glass transition temperature: Basics and
application in pharmaceutical sector“ 2.
[3] B. Wunderlich, Thermal Analysis of Polymeric Materials. Berlin, Heidelberg: Springer-Verlag Berlin Heidelberg, 2005.
[5] G. W. Ehrenstein, G. Riedel und P. Trawiel, Thermal analysis of plastics: Theory and practice. Munich: Hanser, 2004.
[8] G. W. H. Höhne, W. F. Hemminger und H.-J. Flammersheim, Differential Scanning Calorimetry,2. Aufl. Berlin, Heidelberg: Springer, 2003.
[9] DIN 51007:2019-04, Thermische Analyse_(TA)_- Differenz-Thermoanalyse_(DTA) und
Dynamische Differenzkalorimetrie_(DSC)_- Allgemeine Grundlagen, 51007:2019-4, 2019.
