Classification and Mechanical Properties of Synthetic Polymers

Synthetic polymers are classified as elastomers, fibers, or plastics based on their crystallinity. Crystallinity, the degree of long-range order in the solid state, influences the mechanical properties (stretching or contracting) of elastomers. Elastomers are flexible polymers that can expand or contract easily upon the application of an external force. They have numerous crosslinks that pull them back into their original shape when stress is removed. Silicones, for instance, are highly elastic due to the weak directional constraints on silicon-oxygen bonds.

Fibers are polymeric materials with a low degree of branching, allowing the molecules to lie parallel and acquire strength from the interactions between them. One example is nylon-66. Unlike elastomers, fibers need to resist stretching, requiring the chains to be nearly fully extended and have strong interactions between them. Hydrogen bonding between chains can achieve this resistance. Plastics are polymers that can only attain a limited degree of crystallinity and, as a result, are neither as strong as a fiber nor as resilient as an elastomer.

The crystallinity of synthetic polymers can be destroyed by thermal motion at high temperatures, leading to a kind of intramolecular melting from a crystalline solid to a more fluid random coil. This occurs at a specific melting temperature, T_m, which increases with the strength and number of intermolecular interactions in the material. All synthetic polymers undergo a transition from a state of high to low chain mobility at the glass transition temperature, T_g.