7 Polymers

Polymer chains and properties

Polymers are materials made of very large molecules: long chains built from repeating smaller units called monomers. Their properties depend on their chemical composition and on how their chains are arranged and connected. These differences help explain why polymers can be rigid and strong, soft and flexible, or highly elastic.

Chain arrangement and packing

Polymer chains may be linear, branched, or joined into a cross-linked network. Linear chains can often pack closely, while branches tend to hinder packing. Cross-links are chemical connections between chains that restrict their movement. Increasing cross-linking generally makes a polymer less able to flow and dissolve, although its effect on stiffness and toughness depends on the polymer and degree of cross-linking.

Many polymers have both ordered, tightly packed regions and disordered regions. A polymer is described as semicrystalline when some regions are ordered, and amorphous when its chains lack long-range order. Greater chain packing often increases stiffness and can reduce transparency; amorphous regions generally allow more chain movement.

Properties also depend on chain length, side groups, and attractions between chains. For example, stronger attractions between chains can increase strength and stiffness.

Temperature-dependent behavior

Two useful temperatures help describe polymer behavior. The glass-transition temperature, $T_g$, marks a change in the behavior of amorphous portions: below $T_g$, they tend to be hard and glassy; above it, they become more flexible or rubbery. In semicrystalline polymers, the melting temperature, $T_m$, marks the melting of ordered regions.

These transitions help determine a polymer’s useful temperature range and how it can be processed.

Thermoplastics and melt processing

Thermoplastics soften when heated and harden when cooled. Their chains are not permanently linked into a dense network, so, when sufficiently heated, they can move past one another. This behavior makes them suitable for injection molding, extrusion, and thermoforming. Many can be reheated and reshaped, although repeated processing may degrade their properties.

Examples include polyethylene (PE) and polypropylene (PP), used in packaging and containers; polyethylene terephthalate (PET), used in bottles and fibers; and polycarbonate (PC), used where impact resistance and transparency are useful. Thermoplastics range from flexible films to rigid engineering parts.

Thermosets and curing

Thermosets form a cross-linked network as they cure. Curing may be initiated by heat, light, or a chemical reaction. Once cured, the network cannot flow like a thermoplastic melt, so a thermoset generally cannot be remelted and reshaped; at sufficiently high temperatures it may instead degrade.

Thermosets often retain their shape and stiffness at elevated temperatures and can provide good chemical or dimensional stability, depending on formulation. Epoxy is used in strong adhesives, protective coatings, and fiber-reinforced components. Phenolic resins are used in heat-resistant electrical and molded parts. Processing commonly involves shaping or placing the uncured material before or during cure.

Elastomers and recovery

Elastomers can undergo large, mostly recoverable deformation. In an unstretched elastomer, flexible chains are coiled and disordered. Stretching makes the chains more aligned; when the force is removed, they tend to return toward a less ordered, coiled state. A small number of cross-links—or equivalent physical connections—helps stop chains from sliding permanently past one another. Elastomers are typically used above their glass-transition temperature, where their chains are mobile enough for rubber-like behavior.

Natural rubber and silicone rubber are familiar elastomers used in products such as tires, seals, gaskets, and flexible tubing. Elastomer behavior is not limited to thermosets: thermoplastic elastomers (TPEs) combine rubber-like flexibility with the ability to be processed like thermoplastics. Thermoplastic polyurethane is used in flexible footwear components, grips, and protective parts.

How the polymer classes differ

Thermoplastics and thermosets are distinguished primarily by how their structures respond to heat. Thermoplastics soften and can often be reshaped, whereas thermosets cure into chemically cross-linked networks that resist flow and do not remelt. Elastomer describes a rubber-like mechanical response rather than a single heat-response class, so an elastomer can be either a thermoset or a thermoplastic.

These distinctions connect structure and processing to typical behavior and uses:

• Thermoplastics: Mostly separate chains; soften on heating and can often be reshaped. Uses include packaging, bottles, housings, and fibers.

• Thermosets: Chemically cross-linked networks; cure into a shape and do not remelt. Uses include adhesives, coatings, electrical parts, and composite matrices.

• Elastomers: Flexible chains with limited cross-linking or physical connections; stretch and recover. Uses include tires, seals, tubing, and flexible components.

Overall, chain structure, packing, attractions between chains, and cross-linking govern polymer behavior. These structural differences guide processing and help determine which materials suit a particular use.