Everything about The Glass Transition Temperature totally explained
The
glass transition temperature,
Tg, is the temperature at which an
amorphous solid, such as
glass or a
polymer, becomes brittle on cooling, or soft on heating. More specifically, it defines a pseudo
second order phase transition in which a
supercooled melt yields, on cooling, a
glassy structure and properties similar to those of
crystalline materials for example of an
isotropic solid material.
Tg is usually applicable to wholly or partially
amorphous solids such as common
glasses and
plastics (
organic polymers).
Below the glass transition temperature,
Tg, amorphous solids are in a glassy state and most of their joining bonds are intact. In
inorganic glasses, with increased temperature more and more joining bonds are broken by thermal fluctuations so that broken bonds (termed
configurons) begin to form
clusters. Above
Tg these clusters become macroscopic large facilitating the flow of material. In organic polymers, secondary, non-
covalent bonds between the
polymer chains become weak above
Tg. Above
Tg glasses and organic polymers become soft and capable of
plastic deformation without fracture. This behavior is one of the things which make most plastics useful.
It is important to note that the glass transition temperature is a kinetic parameter, and thus
parametrically depends on the melt cooling rate. Thus the slower the melt cooling rate, the lower
Tg. In addition,
Tg depends on the measurement conditions, which are not universally defined.
The bond system of an amorphous material changes its
Hausdorff dimension from Euclidian 3 below
Tg (where the amorphous material is solid), to fractal 2.55±0.05 above
Tg, where the amorphous material is liquid.
Time dependency
Consider a molecular liquid which is slowly cooling down. At a certain temperature, the average
kinetic energy of molecules no longer exceeds the
binding energy between neighboring molecules and growth of organized solid crystal begins. Formation of an ordered system takes a certain amount of time since the molecules must move from their current location to energetically preferred points at crystal nodes. As temperature falls, molecular motion slows down further and, if the cooling rate is fast enough, molecules never reach their destination — the substance enters into dynamic arrest and a disordered, glassy solid (or supercooled liquid) forms. In fact,
Walter Kauzmann has argued that if such an arrest didn't happen, at still lower temperatures a
thermodynamically paradoxical situation would arise, where the undercooled liquid would have to be denser and of a lower enthalpy than the crystalline phase. Such arrest apparently takes place at certain temperature, which is called the
calorimetric ideal glass transition temperature T0c. This means that
glass transition isn't merely a
kinetic effect, for example merely the result of fast cooling of a melt, but there's an underlying
thermodynamic basis for glass formation. The glass transition temperature
Tg →
T0c as
dT⁄dt → 0.
A full discussion of
Tg requires an understanding of mechanical loss mechanisms (vibrational and resonance modes) of specific (usually common in a given material)
functional groups and molecular arrangements. Factors such as
heat treatment and molecular re-arrangement, vacancies, induced
strain and other factors affecting the condition of a material may have an effect on
Tg ranging from the subtle to the dramatic.
Tg is dependent on the
viscoelastic materials properties, and so varies with rate of applied load. The
silicone toy '
Silly Putty' is a good example of this: pull slowly and it flows; hit it with a hammer and it shatters.
In contrast to the melting points of crystalline materials the glass transition temperature is therefore somewhat dependent on the time-scale of the imposed change. To some extent time and temperature are interchangeable quantities when dealing with glasses, a fact often expressed in the
time-temperature superposition principle. An alternative way to discuss the same issue is to say that a glass transition temperature is only truly a point on the temperature scale if the change is imposed at one particular frequency. This is why the ability to modulate the temperature in a
DSC experiment has made determining T
g considerably more precise. Since
Tg is cooling-rate (or frequency) dependent as the glass is formed, the glass transition isn't considered a true
thermodynamic phase transition by many in the field. They reserve this epithet rather for a transition that's sharp and history-independent.
The IUPAC Compendium of Chemical Terminology, 1997, 66, 583 defines the glass transition as a second order
phase transition in which a supercooled melt yields, on cooling, a glassy structure and properties similar to those of crystalline materials for example of an isotropic solid material. Phase transitions are associated with the symmetry breaking. The translation-rotation symmetry in the distribution of atoms and molecules is unchanged at the liquid-glass transition, which retains the topological disorder of fluids. Symmetry changes at glass transition can be viewed when considered not for atoms but for bonds. The disordered material changes its symmetry, namely the
Hausdorff dimension of bonds, from Euclidian 3D below to fractal 2.55±0.05- dimensional above the glass transition temperature.
In
polymers,
Tg is often expressed as the temperature at which the
Gibbs free energy is such that the
activation energy for the cooperative movement of 50 or so elements of the polymer is exceeded. This allows molecular chains to slide past each other when a force is applied. From this definition, we can see that the introduction of relatively stiff chemical groups (such as
benzene rings) will interfere with the flowing process and hence increase
Tg. With thermoplastics, the stiffness of the material will drop due to this effect. This is shown in the figure below. It can be seen that when the glass temperature has been reached, the stiffness stays the same for a while, until the material melts. This region is called the rubber plateau.
Tg can be significantly decreased by addition of
plasticisers into the polymer matrix. Smaller molecules of plasticizer embed themselves between the polymer chains, increasing the spacing and free volume, and allowing them to move past one another even at lower temperatures. The "
new-car smell" is due to the initial
outgassing of
volatile small-molecule plasticizers used to modify interior plastics (for example, dashboards) to keep them from cracking in the cold, winter weather. The addition of nonreactive
side groups to a polymer can also make the chains stand off from one another, reducing
Tg. If a plastic with some desirable properties has a
Tg which is too high, it can sometimes be combined with another in a
copolymer or
composite material with a
Tg below the temperature of intended use. Note that some plastics are used at high temperatures, for example, in automobile engines, and others at low temperatures.
In
glasses (including
amorphous metals and
gels),
Tg is related to the energy required to break and re-form covalent bonds in a somewhat less than perfect (may be regarded as an understatement) 3D lattice of
covalent bonds. The
Tg is therefore influenced by the chemistry of the glass. E.g., add
B,
Na,
K or
Ca to a
silica glass, which have a
valency less than 4 and they help break up the 3D lattice and reduce the
Tg. Add
P which has a valency of 5 and it helps re-establish the 3D lattice, increasing
Tg.
The
Space Shuttle Challenger disaster was caused by rubber O-rings that were below their glass transition temperature on an unusually cold Florida morning, and thus couldn't flex adequately to form proper seals between sections of the two
solid-fuel rocket boosters.
Measurement of Tg for glasses
[[Image:Tgdilatometric.gif|250px|thumb|Determination of T
g for
glasses by
dilatometry. The linear sections below and above
Tg are marked green;
Tg is the temperature at the point of intersection of the corresponding red regression lines., but can also be modeled in the absence of explicit water molecules, suggesting that part of the transition is due to internal protein dynamics.
Vitrification (glass formation below the
melting point) can occur when starting with a liquid such as
water, usually through very rapid cooling or the introduction of agents that suppress the formation of
ice crystals. This is in contrast to ordinary
freezing which results in ice crystal formation. Additives used in
cryobiology or produced naturally by organisms living in
polar regions are called
cryoprotectants. Vitrification technology is being used to
cryopreserve cells,
tissues and
organs for
transplantation.
Glass transition temperature of some materials
These are only mean values, as the glass transition temperature depends on the cooling-ratio, molecular weight distribution and could be influenced by additives.
Note also that for a semi-crystalline material such as Polyethylene that's 60-80% crystalline at room temperature the quoted glass transition refers to what happens to the amorphous part of the material as the temperature is dropped
References and footnotes
- For glass transition temperatures of various resins, see Engineered Materials Handbook—Desk edition. (1995). ASM International. ISBN 0871702835. p. 369.
- For glass transition temperatures of various glasses, see Mazurin, O.V. Handbook of Glass Data. (1993). Elsevier. ISBN 0444816356.
- Prediction of high weight polymers glass transition temperature using RBF neural networks Journal of Molecular Structure: THEOCHEM, Volume 716, Issues 1-3, 7 March 2005, Pages 193-198 Antreas Afantitis, Georgia Melagraki, Kalliopi Makridima, Alex Alexandridis, Haralambos Sarimveis and Olga Iglessi-Markopoulou
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