D12. Discuss the role of a plasticizer in a polymer and the requisites of an effective plasticizer.

A plasticizer separates the polymer chains, and by doing so the intermolecular bond strength decreases, making the resin more plastic (softer). There are two ways to plasticize a resin. The simplest way (external plasticizers) is to introduce molecules between the chains, molecules that do not form strong bonds between the chains. The other way (internal plasticizers) is to attach long pendant groups to the polymer chains. In the latter case, the backbone structure of the main chains will be separated because of the space the pendant groups occupy between the main chains.

Comparing the external and internal plasticizers suggest that the external plasticizer might be leached out and when that happens the polymer chains approaches each others and the plastic becomes harder. This will not happen with the internal plasticizers. From the above we can expect that water diffusing into a resin will act as a plasticizer. This explains why many plastics after storage in water become "softer."  

Examples

Plasticized resins are used as soft-liners under dentures. Externally plasticized resins (contain unbonded molecules mixed with the resin) can leach the plasticizer, causing the liner to become harder. If on the other hand a resin acting as an internal plasticizer is used, the material remains softer over time.