Glass transition temperature
When the glass transition temperature T g is exceeded , a solid glass or polymer changes into a rubbery to viscous state. In the case of amorphous metals , one speaks of the glass transition temperature and in the case of inorganic-non-metallic glasses of the transformation temperature .
Education and characteristics
A glass forms when a liquid cools down faster than crystallization nuclei can form. This happens particularly easily with asymmetrical molecules and viscous liquids . Glasses are z. B. from the in the colloquial language understood under it inorganic glasses - such as window glass - formed, but also from organic glasses such. B. amorphous plastics and even short-chain organic substances that can be supercooled (e.g. 2-methylpentane with a glass transition temperature of below 80 K).
In the case of polymers, the glass transition from the melt to the solid state is based on the "freezing" of chain segments.
In the case of amorphous plastics, the glass transition separates the brittle, energy- elastic area (glass area) below from the soft, entropy- elastic area above ( rubber-elastic area ). The transition to the flow area (area of plastic deformation ) is not abrupt, but continuous.
Semi-crystalline plastics (many common plastics have a crystalline proportion of 10 to 80%) have both a glass transition temperature below which the amorphous phase freezes (accompanied by embrittlement) and a melting temperature at which the crystalline phase dissolves. The melting temperature clearly separates the entropy-elastic area from the flow area.
A glass transition is not a first-order phase transition and is therefore not linked to an exact temperature like the melting point of crystals. The value found varies systematically depending on the time and length scale or movement mode of the molecular dynamics on which the measurement method used (see below) is sensitive. The measurement method used must therefore be specified precisely for each glass transition temperature. However, the deviation is typically only a few Kelvin , as the movement modes are strongly coupled with one another (all freeze in the same temperature range).
In the case of mixtures with only two components, the Gordon-Taylor equation can be used to predict the glass transition temperature. In addition, the Fox equation and, in the case of stronger intermolecular interactions, the Kwei equation are used. The Gordon-Taylor equation can be extended for mixtures with more than two components.
In a more recent model of the glass transition, the glass transition temperature corresponds to the temperature at which the largest openings between the vibrating elements in the liquid matrix become smaller with falling temperature than the smallest cross-sections of the elements or parts of them.
As a result of the fluctuating input of thermal energy into the liquid matrix, the harmonicity of the molecular vibrations is constantly disturbed and temporary cavities (“free volume”) arise between the elements, the number and size of which depend on the temperature. The glass transition temperature Tg 0 defined in this way is a fixed material constant of the disordered (non-crystalline) state that is only dependent on the pressure. As a result of the increasing inertia of the molecular matrix as Tg 0 is approached , the establishment of the thermal equilibrium is successively delayed, so that the usual measuring methods for determining the glass transition temperature in principle produce too high Tg values. In principle, the following applies: The slower the temperature change rate is set during the measurement, the closer the measured Tg value approaches Tg 0 .
The glass transition temperature can u. a. can be measured using the following methods:
- the dynamic mechanical analysis (DMA); A strong change in the modulus of elasticity and G as well as a pronounced maximum change in damping is observed in a narrow temperature range. Round-robin tests show that the glass transition temperatures of PMMA and PC measured with DMA have a standard measurement uncertainty of approx. 5 to 6 ° C.
- the differential scanning calorimetry (DSC); the heat capacity C p is recorded as a function of the temperature; the heat capacities above and below the glass transition differ, with a continuous transition near the glass transition temperature. The determined glass transition temperature depends very much on the heating or cooling rate: with slow heating or cooling, the values from the heating or cooling process approach each other, but the heat capacity is increasingly difficult to measure at a low rate. A glass transition temperature between 90 and 190 ° C measured with DSC has a standard measurement uncertainty of approximately 1.4 to 2 ° C.
- the dielectric relaxation spectroscopy .
- the dilatometry because the expansion coefficient changes at the glass transition.
Weak intermolecular interactions and thus the glass transition temperature play an important role in food chemistry . Substances dissolved or suspended in water are often measured. During evaporation, the dissolved or suspended substance molecules are brought into close proximity to one another and thus temporarily brought into a glass-like state below their melting point. This state is influenced by additives, which are called either vitrifiers or plasticizers , depending on whether they increase or decrease the glass transition temperature . A further increase in temperature leads to the melt as a result of the dissolution of the weak bonds. With decreasing viscosity, the tendency towards chemical and enzymatic reactions increases, which leads to faster deterioration of food. For a longer shelf life of a food it is therefore necessary to store it below the glass transition temperature . The texture of ready meals and the solubility of instant soups and other powdery foods can also be influenced with the help of this parameter.
Application temperature of plastics
Whether a plastic can be used above or below its glass transition temperature depends on the type of plastic (it should be noted that the glass transition temperature of a plastic or elastomer increases with its crosslinking density, i.e. the glass transition temperature of a thermoset is significantly higher than that of a thermoset Thermoplastics):
- Amorphous thermoplastics can only be used below the glass transition temperature. Processing usually takes place above this.
- Semi-crystalline thermoplastics are used both below and above the glass transition temperature. Semi-crystalline thermoplastics, the glass transition temperature of which is higher than their application temperature (e.g. polyethylene terephthalate ), tend to be stiff and soften to different degrees during the glass transition (depending on the degree of crystallinity). Semi-crystalline thermoplastics whose glass transition temperature is below the application temperature (e.g. polyethylene ), on the other hand, are also relatively soft at the application temperature and become brittle if the glass transition temperature is not reached. In both cases, use above the melting temperature does not make sense.
- Elastomers are generally used in the rubber-elastic range, i.e. above the glass transition temperature. Below the glass transition temperature they become very brittle, which means that their use is not advisable. Thus, for example, as a cause of the accident of the Space Shuttle Challenger an O-ring - sealing of fluoroelastomer determined, which was operated below its glass transition temperature where it insufficiently elastic was not dense and thus held. The upper temperature limit of these materials is their respective decomposition temperature.
- Duroplasts are used both below and above the glass transition temperature. Thermosets, whose glass transition temperature is below room temperature, are, however, to be counted among the elastomers. The upper temperature limit of thermosets is their respective decomposition temperature.
Use temperature of glasses
In practice, glass is never used above T g . If glass is exposed to temperature fluctuations with a peak above T g , when these peaks cool down, stresses occur in the glass, which typically lead to breakage quickly. After production, glass must pass through the temperature range around T g by means of slow cooling. This minimizes tension.
As a rule, glass or glass components may not be loaded up to T g . T g lies within the so-called transformation range, the lower limit of which is described by the lower cooling temperature . This temperature represents the theoretical maximum temperature of a type of glass. In practice, this temperature is always 50-100 ° C below T g .
For borosilicate glasses and soda-lime glasses, the T g is around 500 ° C, which is significantly higher than for most plastics. Lead glasses are a little lower at around 400 ° C. Aluminosilicate glasses are significantly higher at around 800 ° C.
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