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Created page with "{{Language_sel|LANG=ger|ARTIKEL=Glastemperatur}} {{PSM_Infobox}} Glass transition temperature __FORCETOC__ ==Terminology== The glass transition temperature ''T''g 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. Inorgan..."

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{{Language_sel|LANG=ger|ARTIKEL=Glastemperatur}}
{{PSM_Infobox}}
Glass transition temperature
__FORCETOC__

==Terminology==

The glass transition temperature ''T''g 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 [[Thermoplastic Material|thermoplastics]] or the amorphous regions in semi-crystalline thermoplastics, most [[Thermosets|duromers]] and many [[Elastomers|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 Elasticity|energy-elastic state]] to an [[Entropy Elasticity|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|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'' < ''T''g) 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 [[Polymer|polymeric]] [[Material & Werkstoff|materials]]:

{| border="1px" style="border-collapse:collapse"
!! style= "width:300px; background:#DCDCDC" | material group
!! style= "width:300px; background:#DCDCDC" | application area
!! style= "width:300px; background:#DCDCDC" | explanation
|-
|amorphous homopolymer [[Thermoplastic Material|thermoplastics]]
|below the glass transition temperature
|otherwise too soft and elastic or too viscous
|-
|semi-crystalline homopolymer [[Thermoplastic Material|thermoplastics]]
|above the glass transition temperature
|otherwise too brittle
|-
|[[Thermosets|thermosets]]
|below and obove glass transition temperature
|only a minor influence of temperature on [[Tensile Strength|strength]] and [[Stiffness|stiffness]] due to high cross-linking density
|-
|[[Elastomers|elastomers]]
|above the glass transition temperature of the amorphous rubber phase
|otherwise hard and brittle, without [[Entropy Elasticity|entropic elastic]] properties
|-
|rubber modified [[Thermoplastic Material|thermoplastics]] and [[Thermosets|thermosets]]
|above the glass transition temperature of the amorphous rubber phase
|otherwise, [[Deformation Mechanisms|mechanisms]] that increase [[Toughness|toughness]] through [[Deformation|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)|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.

[[Elastic Modulus#Dynamic-mechanical analysis (DMA)|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 [[Thermal Expansion Coefficient|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|Rubber elasticity]]
* [[Dynamic-mechanical Analysis (DMA) – General Principles|Dynamic-mechanical analysis (DMA) – General principles]]
* [[Shrinkage Test|Shrinkage test]]
* [[Brittle-Tough Transition|Brittle-tough transition]]
* [[Curing]]
* [[Toughness Temperature Dependence|Toughness temperature dependence]]
* [[Cross-linking Elastomers|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)

[[Category:Thermoanalytical Methods]]

Glass Transition Temperature - Revision history