An Example
Poly(ethylene terephthalate) (PET) is a polymer which is widely used in packaging, fiber, and engineering applications. This polymer may be synthesized by a variety of means, including the reaction of ethylene glycol with dimethyl terephthalate, as illustrated below:

The molecular weight is built by reaction in the melt state until the polymer has an inherent viscosity of approximately 0.5. It is then extruded and pelletized. The material at this molecular weight may be used directly in fiber or textile applications. For most packaging and engineering applications the molecular weight is further increased through polymerization in the solid state.

The shaping of the polymer through a variety of potential processing operations; fiber spinning, blow molding, thermoforming, injection molding, etc.; is where classical polymer processing starts.

Injection Molding of Poly(ethylene terephthalate)
The mechanical properties of a polymer are generally not a material function, but instead depend on the manner in which it was processed. Semicrystalline polymers such as PET are excellent examples of material which are sensitive to processing conditions.

The same poly(ethylene terephthalate) was injection molded under slightly different conditions of melt temperature and mold temperature. The part produces is a standard rectangular shape, 6 inches long, 1/2 inch wide, and 1/8 inch thick. The influence of these changes on the mechanical properties are summarized below.

How is it possible that such seemingly minor changes in the process conditions resulted in large changes in the material properties?

In what specific ways does the processing control the polymer microstructure in this particular case? This is an excellent example of why it is important to understand polymer processing and its interaction with the material.