Polymers are giant chains of molecules formed by the process called polymerisation. They are characterised by low strength and Young's modulus, low cost and density, excellent corrosion resistance but poor mechanical properties at elevated temperatures. Engineering polymers posses all of the above qualities but are designed for better performance and improved strength at higher temperatures where ordinary polymers fail. They are split into three different groups:
The main difference between the three groups of polymers is the way their molecule chains are structured. In other words, the amount and strength of cross linking.
The chains may be tens of thousands of monomers long and because of the change of orientation of carbon atoms along these chains, the configurations formed by them are complex and random. This is known as an amorphous polymer.
Thermoplastics are best known for their ability to soften and exhibit viscous flow properties when heated. In other words, they can be melted as opposed to thermosetting polymers. This is due to the fact that there is no covalent bonding between the polymer chains which gives them the ability to rotate and slide.
When cooled from their molten state, thermoplastics can remain amorphous or they may become partially crystalline (pictured on the below).
This means that the chains closely align over long distances in straight lines, loop around and stack on top of each other. Each loop is about 100 carbon atoms long and because the folds are in 3D, they form plates or what it called lamellae.
These types of thermoplastics take various form with the most common one being spherulitic. As the chains in liquid are structured into more orderly configurations, the density naturally increases as there is less empty space between them. In addition to this, crystalline structures increase the strength in a similar manner to dispersion strengthening in metals.
Thermosetting polymers are composed of long, rigid cross-linked chains (pictured in red on the diagram below) of molecules and as a result they are a lot stronger but also more brittle than thermoplastics and less ductile.
This is due to the fact that chains cannot rotate or slide past each other. They don't have a specific melting temperature (in fact, they don't melt at all) and once they have been cross linked they cannot be easily reprocessed.
They are formed by a non-reversible, two part chemical
reaction: liquid resin + reactive liquid chemicals
(Chemical cross linking) Hard rigid polymer.
An optional post curing step may be employed by the use of a higher temperature
to make a fully cured polymer. It is usually achieved at temperature between 100-180°C
for one to several hours.
Elastomers are in the middle of thermoplastics and thermosetting polymers in respect to their cross-linkage. They have an intermediate amount of cross-linking between their molecular chains (pictured in red on the diagram below) which gives them exceptionally high elastic properties (high resilience). The organic chain molecules uncoil as load is applied while the cross-links between chains provide elasticity once all chains have been untangled.
The stress-strain curve for elastomers is non-linear because it represents constant elastic deformation. Initially, the modulus of elasticity gets smaller as the chains uncoil. Once the chains have uncoiled, further elasticity occurs by the streching of cross-links between the chains, leading to a higher modulus of elasticity.
Common types of elastomers are: polyisoprene (tires), polybutadiene (industrial tires, vibration mounts), polyisobutylene (piping, insulation, coatings), polychloroprene [Neoprene] (hoses, cable sheathing), silicone (gaskets, seals).
A special group of polymers that do not rely on cross-linking to produce large amount of elastic deformation are thermoplastic elastomers. Approximately 25% of the chains are made up of styrene. The styrene ends of several chains form spherical-shaped domains. The domains are string and rigid and tightly hold the chains together because styrene has a high glass transition temperature. Butadiene repeat units are contained between styrene domains and because they posses a glass transition temperature below room temperature, they behave in a soft, rubbery manner. The polymer has elastic properties due to recoverable movement of the chains, however sliding of the chains at normal temperatures is prevented by styrene domains.
Askeland D. R. 1996, The Science and Engineering of Materials, 3rd S.I. edn, Chapman & Hall, London