How Plastics Are Made(2)-The Structure of Polymers

As we have discussed, polymers can be homopolymers or copolymers. If the long chains show a continuous link of carbon-to-carbon atoms, the structure is called homogeneous. The long chain is called the backbone. Polypropylene, polybutylene, polystyrene and polymethylpentene are examples of polymers with homogeneous carbon structure in the backbone. If the chains of carbon atoms are intermittently interrupted by oxygen or nitrogen, the structure is called heterogeneous. Polyesters, nylons, and  polycarbonates are examples of polymers with heterogeneous structure. Heterogeneous polymers as a class tend to be less chemically durable than homogeneous polymers although examples to the contrary are numerous.

Different elements can be attached to the carbon-to-carbon backbone.  Polyvinyl chloride (PVC) contains attached chlorine atoms. Teflon contains attached fluorine atoms.

How the links in thermoplastics are arranged can also change the structure and properties of plastics. Some plastics are assembled from monomers such that there is intentional randomness in the occurrence of attached elements and chemical groups. Others have the attached groups occur in very predictable order. Plastics will, if the structure allows, form crystals. Some plastics easily and rapidly form crystals, such as HDPE—high density polyethylene. HDPE can appear hazy from the crystals and exhibits stiffness and strength. Other plastics are constructed such that they cannot fit together to form crystals, such as low density polyethylene, LDPE. An amorphous plastic typically is clear in appearance. By adjusting the spatial arrangement of atoms on the backbone chains, the plastics manufacturer can change the performance properties of the plastic.

The chemical structure of the backbone, the use of copolymers, and the chemical binding of different elements and compounds to a backbone, and the use of crystallizability can change the processing, aesthetic, and performance properties of plastics. The plastics can also be altered by the inclusion of additives.

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