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About polymers

About polymers

Polymers, macromolecules, can be grouped along different principles. The following is an attempt to provide a quick overview and background to these materials, to highlight some common features as well as major differences between them. Some noteworthy technical terms are also explained.

Most of the engineering plastics are thermoplastic materials, linear macromolecules without chemical crosslinks. In amorphous materials, the molecular structure is random and isotropic, in (partially) crystalline materials there will be areas with molecular orientation and packing, leading to (generally) positive changes in properties. Themoplastic materials, when heated, go from a solid glass-like state, via a soft rubber-like state to a molten fluid-like state. The macromolecules are internally mobile, and the mobility is primarily controlled by the application of energy. These phase transitions are reversible, and the materials can theoretically be physically transformed endlessly. In reality though, the materials will be thermo-oxidatively degraded, leading to unacceptable loss of properties.

By creating chemical bonds (crosslinks) between different macromolecules, a network is created which cannot be reversibly thermally transformed. In rubber, the material is lightly crosslinked, with visco-elastic properties, in thermosets, the crosslink density is high and the molecular mobility small. As no real melting points exist, the materials will eventually carbonize, and they cannot technically be recycled or reshaped.

Some thermoplastics have elastomeric properties. These materials have not chemical crosslinks, but physical, which lead to rubber-like properties. They are called elastomers or thermoelastomers, and are thermoplastics with defined melting point and reversible processability.

Two main principles can be identified in polymerisation. In addition polymerisation, molecules (monomers) are added stepwise to a growing chain until wished molecular weight is achieved. Different addition mechanisms are with organometallic catalyse, radical polymerisation, anionic or cationic polymerisation, ring-opening reactions etc. The propagating chain contains the same atoms as the monomer, and the reaction product is the sum of the reacted monomers.

In condensation polymerisation, different monomers are reacted. The repeating unit of the reaction product contains fewer atoms than the monomers, and a rest product, often water, has been released.

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