Glass transition temperature
From Freepedia
A simplistic view of a material's glass transition temperature (Tg) is the temperature below which molecules have very little mobility. On a larger scale, polymers are rigid and brittle below their glass transition temperature and can undergo plastic deformation above it. The Tg is usually applicable to amorphous phases and is commonly applicable to glasses and plastics.
Consider a molecular liquid, which is cooling slowly down. At certain temperature the kinetic energy of molecules no longer surpasses the binding energy between neighboring molecules and growth of organized solid matter, a crystal, begins. Formation of an ordered system takes certain amount of time since molecules must move from their current location to energetically preferred point at crystal nodes. As temperature falls molecular motion slows further down and if cooling rate is fast enough molecules never reach their destination - the substance enters into dynamic arrest and disordered solid - a glass forms. Such arrest apparently takes place at certain temperature, which is called the glass transition temperature, Tg. Needless to say Tg is cooling rate dependent as is the glass so formed.
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 (silly putty is a good example of this, as is stiff cornflour/water mixtures - pull slowly and they flow, pull rapidly and they shatter).
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 side chains and relatively stiff chemical groups (such as benzene rings) will interfere with the flowing process and hence increase Tg.
Tg can be significantly decreased by addition of plasticizers into the polymer matrix. Smaller molecules of plasticizer embed themselves between the polymer chains, space them apart (increasing the free volume) and allow them to move against each other easier.
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. Eg. 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 a rubber O-ring that was below its glass transition temperature and thus could not flex adequately to form a proper seal around one of the two solid rocket boosters.
Biophysics
Proteins also possess a glass transition temperature below which both anharmonic motions and long-range correlated motion within a single molecule quenched. The origin of this transition is primarly due to "caging" by glassy water 1, but can also be modeled in the absence of explicit water molecules, suggesting that part of the transition is due to internal protein dynamics. 2
Glass transition temperature of some materials:
| Polymer | Tg (oC) |
|---|---|
| Polyethylene (LDPE) | -125 |
| Polypropylene (atactic) | -20 |
| Poly(vinyl acetate) (PVAc) | 28 |
| Poly(ethyleneterephthalate) (PET) | 69 |
| Poly(vinyl alcohol) (PVA) | 85 |
| Poly(vinyl chloride) (PVC) | 81 |
| Polypropylene (isotactic) | 100 |
| Polystyrene | 100 |
| Poly(methylmethacrylate) (atactic) | 105 |
References
- 1 Vitkup D, Ringe D, Petsko GA, Karplus M (2001). Solvent mobility and the protein 'glass' transition. Nature Structural Biology 7: 34–38.
- 2 Salsbury FR, Han WG, Noodleman L, Brooks CL (2003). Temperature-dependent behavior of protein-chromophore interactions: A theoretical study of a blue fluorescent antibody. CHEMPHYSCHEM 4: 848–855.
