Glass transition temperature

The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). A material’s glass transition temperature, Tg, is the temperature below which molecules have little relative mobility. Tg is usually applicable to wholly or partially amorphous phases such as glasses and plastics. For inorganic or mineral glasses, such as common silicon dioxide (SiO2) glass, it is the mid-point of a temperature range in which they gradually become more viscous and change from being liquid to solid. Thermoplastic (non-crosslinked) polymers are more complex because, in addition to a melting temperature, Tm, above which all their crystalline structure disappears, such plastics have a second, lower Tg below which they become rigid and brittle, and can crack and shatter under stress. Small molecular weight pure substances such as water have just one such condensed-phase temperature, below which they are solid crystals (or amorphous ice if cooled below Tg fast enough) and above which they are liquids.

Above Tg, the secondary, non-covalent bonds between the polymer chains become weak in comparison to thermal motion, and the polymer becomes rubbery and capable of elastic or plastic deformation without fracture. This behavior is one of the things which make most plastics useful. But such behavior is not exhibited by crosslinked thermosetting plastics which, once cured, are set for life and will shatter rather than deform, never becoming plastic again when heated, nor melting.

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, Kauzmann has argued that if such an arrest did not 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 is not merely a kinetic effect, i.e. merely the result of fast cooling of a melt, but there is 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 Tg considerably more precise. Since Tg is cooling-rate (or frequency) dependent as the glass is formed, the glass transition is not considered a true thermodynamic phase transition by many in the field. They reserve this epithet rather for a transition that is 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 e.g. 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 (e.g., 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, e.g., 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 could not flex adequately to form proper seals between sections of the two solid-fuel rocket boosters.

Biophysics
Proteins also possess a glass transition temperature below which both anharmonic motions and long-range correlated motion within a single molecule are quenched. The origin of this transition is primarily due to "caging" by glassy water, 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 is 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