Glass Transition Temperature
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Glass transition temperature
Terminology
The glass transition temperature Tg is closely linked to the concept of the glass state. Contrary to what is often mistakenly assumed, the glass state is by no means equivalent to a super-cooled (metastable) melt, but rather represents a specific unstable state of solids. Inorganic glasses include silicate and metallic glasses, as well as some other low-molecular-weight substances. The group of organic glasses comprises amorphous thermoplastics or the amorphous regions in semi-crystalline thermoplastics, most duromers and many elastomers or their amorphous phases.
The glass transition temperature, often referred to as the glass transition point, is the temperature at which an inorganic or organic glass, when heated, transitions from a glassy state to a liquid to rubbery state, depending on the material (with the exception of duromers). In organic glasses in particular, the glass transition is associated with a transition from an energy-elastic state to an entropy-elastic state, which is followed by a highly viscous liquid state in amorphous thermoplastics. Conversely, as the temperature decreases, the glass state can be reached at the glass transition temperature from an initial liquid to rubbery state.
Regardless of the typical values given in scientific publications, material data sheets and databases, the glass transition temperature is not a constant for a specific material; rather, apart from the method used to determine it, it depends to a considerable extent on the velocity of temperature increase or decrease. Furthermore, the time and temperature during which the material remains in the glass state influence the glass transition temperature.
Setting limits for use
As the material is in an unstable state in the glassy state, it tends towards a state of equilibrium (prehistory), with the result that the glass transition temperature shifts to lower values as both time and temperature (T < Tg) increase. Heating a glassy material above the glass transition temperature erases the material’s history. The instability of the glassy state, the irreversibility of the glass transition and other thermodynamic characteristics are clear indications that the glass transition is not a phase transition.
Despite the restriction outlined above—that the glass transition temperature is, strictly speaking, not a material parameter—it is of immense importance in determining the service limits of polymeric materials:
| material group | application area | explanation |
|---|---|---|
| amorphous homopolymer thermoplastics | below the glass transition temperature | otherwise too soft and elastic or too viscous |
| semi-crystalline homopolymer thermoplastics | above the glass transition temperature | otherwise too brittle |
| thermosets | below and obove glass transition temperature | only a minor influence of temperature on strength and stiffness due to high cross-linking density |
| elastomers | above the glass transition temperature of the amorphous rubber phase | otherwise hard and brittle, without entropic elastic properties |
| rubber modified thermoplastics and thermosets | above the glass transition temperature of the amorphous rubber phase | otherwise, mechanisms that increase toughness through deformation will not take effect |
Methods for determining glass temperature
Several methods have become established for determining the glass temperature:
Dynamic Differential Scanning Calorimetry (DSC) exploits the fact that the heat capacity changes almost abruptly as a function of temperature when passing through the glass transition region. Due to the velocity dependence of the glass transition, measurements are usually carried out at low heating rates of approximately 10 K/min. The glass transition traditionally detected by DSC is also referred to as the thermal glass transition, as it is based on thermal effects.
Dynamic mechanical analysis (DMA) exploits the fact that molecular mobility in the glass transition region increases significantly as the temperature rises, a phenomenon manifested, amongst other things, by a peak in the mechanical loss factor tan δ. As the glass transition detected by DMA is underpinned by molecular dynamic effects, it is also referred to as a dynamic glass transition.
The determination of the glass transition temperature using dilatometry is based on the fact that the coefficient of thermal expansion is lower in the glass state than at temperatures above the glass transition temperature.
The imaginary part of the dielectric constant, as determined by dielectric relaxation spectroscopy, reaches a maximum at the glass transition temperature.
See also
- Time–temperature shift law
- Elasticity
- Rubber elasticity
- Dynamic-mechanical analysis (DMA) – General principles
- Shrinkage test
- Brittle-tough transition
- Curing
- Toughness temperature dependence
- Cross-linking elastomers
References
- Donth, Ernst-Joachim: The Glass Transition – Relaxation Dynamics in Liquids and Disordered Materials. Springer, Berlin Heidelberg New York (2001) (ISBN 978-3-540-41801-6)