Classification of Polymers

Thermoplastic Polymers
These are linear, one-dimensional polymers which have strong intramolecular covalent bonds and weak intermolecular van Der Waals bonds. At elevated temperature, it is easy to "melt" these bonds and have molecular chains readily slide past one another. These polymers are capable of flow at elevated temperatures, can be remolded into different forms, and in general, are dissolvable. A thermoplastic, under the application of appropriate heat, can be melted into a "liquid" state.

Thermosetting Polymers
These are three-dimensional amorphous polymers which are highly crosslinked (strong, covalent intermolecular bonds) networks with no long-range order. Thermosetting polymers are those resins which are "set" or "polymerized" through a chemical reaction resulting in crosslinking of the structure into one large 3-dimensional molecular network. Once the chemical reaction or polymerization is complete, the polymer becomes a hard, infusible, insoluble material which cannot be softened, melted or molded non-destructively.

Rubbers and Elastomers
 Rubber must be a high polymer (polymers with very long chains) as rubber elasticity, from a molecular standpoint, is due to the coiling and uncoiling of very long chains. To have "rubber-elastic properties" a rubber materials' use temperature must be above its glass transition temperature and it must be amorphous in its unstretched state since crystallinity hinders coiling and uncoiling. Rubbers are lightly crosslinked in order to prevent chains from slipping past one another under stress without complete recovery.

Fibers
 A fiber is often defined as having an aspect ratio (length/diameter) of at least 100. Stresses and strength of fibers are reported in terms of tenacity in units of grams/denier. In melt spinning, polymer pellets are gravity fed into an extruder and subjected to shear loading at elevated temperatures. A molten stream of polymer is forced by pressure through shaped holes and stretched into a solid state. Then the polymer is streched to have molecular alignment along the axial direction and crystallized in a preferred direction so that no spherulites form.

Liquid Crystals
The structure of liquid crystals such as Spectra 1000 is unique. It is a near-ideal in structure with most of its molecules virtually stretched out. Continuous crystals are readily attained in liquid crystalline polymers, as the molecules are already aligned in parallel positions in the melt whereas the continuous morphology of PE would require elaborate processing to avoid chain entanglement and chain folding.

Polymer Synthesis

1. Starting with tri functional (or higher) monomers
2. Chemically creating crosslinks from previous linear polymers

Chain Growth
Chain growth polymerization involves an active chain site which reacts wiht an unsatureated (or heterocyclic) monomer such that the active site is recovered at the chain end. In general, high molecular weight species are made by the successive addtion of new monomers to the chain end. There are several different techniques of synthesis depending on the active site. These are: