The higher the crystallinity, the more stable the material. The manufacturing process used with plastic materials is called polymerization. In the polymerization process billions of small molecules, called monomers are combined to form giant molecules called polymers. These new giant molecules form microstructure that are linear, branched, or cross linked. Linear molecules form into crystalline structures.
Branched molecules form into amorphous structures.
Cross linked molecules will not melt and create thermoset plastic materials.