List the two molecular characteristics that are essential for elastomers.

Long-chain polymer molecules are the fundamental structural units of elastomers, providing them with their characteristic stretchability and flexibility. These molecules are chains composed of repeating units called monomers, linked together like a string of beads. Imagine a spaghetti noodle; it's long and can be twirled and twisted, much like the polymer chains that make up an elastomer.

The length of these molecules plays a critical role in how an elastomer behaves. The longer the chains, the more they can extend under tension. This ability to stretch is a result of the molecular segments sliding past each other when force is applied. Once the force is removed, the chains recoil back to their original, entangled state, much like how a relaxed rubber band retracts after being stretched.

It's this interplay of flexibility and retraction in the long polymer chains that permits elastomers to be used in diverse applications, from the soles of your shoes to the seals around your car windows. They are designed to endure significant deformation without breaking, thus serving their purpose effectively in everyday products.

Cross-linking in polymers is a chemical process that significantly affects the elastomers' mechanical properties. If we go back to the spaghetti analogy, cross-linking would be like placing random spots of glue between the noodles. The noodles could still move, but they would be connected at various points, providing a limited structure to the otherwise chaotic tangle.

In elastomers, cross-links are connections between the long polymer chains, which restrict their motion to a certain extent; this is crucial for the material to 'snap back' to its original form. These links can be formed via covalent bonds, which are strong and permanent, or through weaker, reversible interactions like hydrogen bonds or ionic interactions.

Cross-linking is what makes the difference between a liquid plastic that flows freely and the tough, resilient properties of a rubber band. The density and type of cross-links can be altered to fine-tune the elastomer's properties, adjusting its hardness, elasticity, and resistance to chemicals and temperature. Understanding the relationship between cross-linking and the properties of the material is essential in designing elastomers for specific applications.

Elasticity and resilience are the hallmark traits of elastomers, allowing them to be deformed and then return to their original shape without permanent damage. These properties go hand in hand, but they are not the same. Elasticity is the ability of a material to stretch and change shape when a force is applied. A material must be able to stretch to be elastic, but how well the material can return to its original state after being deformed is a measure of its resilience.

Imagine pressing a ball made of soft clay versus one made of rubber. The clay might change shape easily (showing it's malleable), but it won't bounce back like the rubber ball. This difference illustrates resilience; the rubber ball has higher resilience than the clay.

The resilience of elastomers comes from their molecular makeup—long-chain molecules bound by cross-links. These features enable elastomers to absorb and dissipate energy from impacts or deformations, making them ideal for shock absorbers and vibration dampeners. By carefully balancing the degree of cross-linking and the flexibility of the polymer chains, manufacturers create materials that can withstand various degrees of stress, from the soft rubber of a silicone spatula to the firm, durable tire of a car.