Polymers come in various categories, each with unique properties. Thermoplastics can be reshaped, thermosets are permanently set, elastomers are stretchy, and fibers are strong and thin. These differences stem from their molecular structure and processing methods.
The structure of polymers greatly influences their properties. Factors like crystallinity, cross-linking, molecular weight, and tacticity affect characteristics such as strength, flexibility, and melting point. Understanding these relationships is key to designing polymers for specific applications.
Polymer Categories and Physical Properties
Categories of polymers
- Thermoplastics soften and melt when heated, harden when cooled allowing them to be reshaped and remolded multiple times (polyethylene, polypropylene, PVC)
- Thermosets undergo irreversible chemical cross-linking during molding preventing them from being reshaped or remolded once set (epoxy resins, polyurethanes, vulcanized rubber)
- Elastomers are highly elastic and flexible materials that can be stretched and return to their original shape (natural rubber, silicone rubber, spandex)
- Fibers are long, thin, and strong polymers with high tensile strength and orientation (nylon, polyester, Kevlar)
Polymer Structure and Properties
Structure vs properties of polymers
- Crystallinity refers to the degree of ordered molecular arrangement where higher crystallinity leads to increased hardness and higher melting points while amorphous regions contribute to elasticity and flexibility
- Cross-linking involves chemical bonds formed between polymer chains that increase hardness, strength, and resistance to heat and solvents but reduce elasticity and ability to be reshaped
- Molecular weight represents the average length of polymer chains where higher molecular weight increases strength, viscosity, and melting point
- Tacticity describes the regularity of side group arrangement along the polymer chain where isotactic and syndiotactic polymers have higher crystallinity than atactic polymers
- Stereochemistry of the polymer chain influences properties such as crystallinity, melting point, and mechanical strength
Polymer Chain Characteristics
- Polymer chains are long molecules composed of repeating units called monomers
- The degree of polymerization refers to the number of monomer units in a polymer chain, affecting properties like strength and viscosity
- Copolymers are formed from two or more different types of monomers, allowing for tailored properties
- Glass transition temperature is the temperature at which an amorphous polymer transitions from a hard, glassy state to a soft, rubbery state
- Viscoelasticity is a property of polymers that exhibit both viscous and elastic characteristics when undergoing deformation
Fiber formation and polymer strength
- Spinning involves extruding a polymer melt or solution through small holes (spinneret) followed by rapid cooling or solvent evaporation to solidify the polymer into filaments
- Drawing stretches the filaments to several times their original length aligning polymer chains along the fiber axis and increasing crystallinity and orientation
- Heat treatment further increases crystallinity and strength by heating the fibers under tension
The resulting fibers have:
- High tensile strength along the fiber axis
- Anisotropic properties meaning different properties in different directions
- Increased modulus and reduced elasticity compared to the bulk polymer