Classification and Mechanical Properties of Synthetic Polymers

Synthetic polymers are classified into elastomers, fibers, and plastics based on their mechanical properties and crystallinity. Elastomers like silicones are highly flexible and return to their original shape after stress is removed. Fibers like nylon-66 exhibit high tensile strength and resist stretching. Plastics contain both crystalline and amorphous regions, making them neither as strong as fibers nor as resilient as elastomers.

Elastomers contain numerous crosslinks between polymer chains that allow the material to stretch or compress significantly under mechanical stress. When stress is removed, these crosslinks pull the chains back into their original shape, restoring the material's structure. This elasticity arises from the crosslinks' ability to maintain chain connectivity while permitting temporary deformation without permanent damage.

Fibers like nylon-66 have low branching, allowing polymer chains to align parallel to each other. This parallel arrangement enables strong attractions between neighboring chains, including hydrogen bonding, which provides high tensile strength and resistance to stretching. The nearly fully extended chains and strong intermolecular interactions make fibers resistant to deformation under applied force.

Plastics contain both crystalline regions, which provide strength and structural order, and amorphous regions, which allow flexibility. As temperature increases, crystalline regions lose their ordered arrangement while amorphous regions transition from a rigid, glass-like state to a softer state. This combination gives plastics moderate strength and flexibility, positioning them between fibers and elastomers.

The glass transition temperature, or Tg, is the point at which all synthetic polymers undergo a transition from high to low chain mobility. Below Tg, polymers behave rigidly; above it, chains move more freely, making the material softer and more flexible. This temperature depends on the polymer's structure and intermolecular interactions.

The melting temperature, or Tm, is the specific temperature at which crystalline regions lose their ordered arrangement and the polymer transitions from a crystalline solid to a more fluid random coil. Tm increases with the strength and number of intermolecular interactions in the material. Polymers with stronger hydrogen bonding or other interactions have higher melting temperatures.

Crystallinity, the degree of long-range order in the solid state, directly influences mechanical properties like stretching and contracting. Higher crystallinity increases strength and structural rigidity, as seen in fibers. Lower crystallinity allows more flexibility, as in elastomers and plastics. Understanding molar masses polymers helps characterize how chain length affects crystallinity and resulting mechanical behavior.