Mechanical Properties of Polymers

Polymers respond to stress in a way that is intermediate between elastic solids and viscous liquids, so their stiffness, strength, and toughness depend on temperature, time, and rate, and are described by viscoelasticity and rubber elasticity.

Definition

The mechanical properties of polymers are their stiffness, strength, deformability, and energy absorption under load, which arise from viscoelastic behavior combining solid-like elasticity and liquid-like flow and depend on temperature, rate, and molecular structure.

Scope

This topic covers the mechanical behavior of polymers: linear viscoelasticity and the storage and loss moduli, creep and stress relaxation, the time-temperature equivalence of mechanical response, rubber elasticity of crosslinked networks, yielding, drawing, and fracture, and how molar mass, crystallinity, crosslinking, and temperature jointly determine ultimate properties.

Core questions

• Why is polymer mechanical response time- and temperature-dependent?
• How do storage and loss moduli describe viscoelastic behavior?
• What molecular origin gives rubber its entropic elasticity?
• How do molar mass, crystallinity, and crosslinking control strength and toughness?

Key theories

Linear viscoelasticity

Polymer response to stress combines elastic and viscous contributions, captured by frequency-dependent storage and loss moduli and by creep and stress-relaxation functions, and unified across temperatures through time-temperature superposition.

Entropic theory of rubber elasticity

The restoring force of a stretched crosslinked rubber is entropic, arising from the reduced conformational entropy of extended network chains, so the modulus is proportional to crosslink density and increases with absolute temperature.

Mechanisms

Below the glass transition a polymer is a stiff glass that can fail in a brittle manner; above it, amorphous chains are mobile and the material is rubbery or, if uncrosslinked and above any melting, flows. Entanglements give melts and solids a transient elastic network, while permanent crosslinks give true rubber elasticity governed by entropy. Under load, polymers can yield by shear banding or crazing, draw to align chains and strengthen in the draw direction, and ultimately fracture; the balance among these processes, set by molar mass, crystallinity, crosslinking, and rate, determines whether a material is brittle or tough.

Clinical relevance

Mechanical-property control is the basis of polymer engineering: rubber elasticity enables tires, seals, and elastomers; high modulus and strength from orientation and crystallinity enable fibers and films; and toughening through rubber phases or controlled crazing enables impact-resistant plastics. Viscoelastic analysis guides design against creep, fatigue, and temperature- and rate-dependent failure.

History

The kinetic theory of rubber elasticity, relating modulus to network chains and to entropy, was developed in the 1940s and codified by Treloar and Flory; the systematic treatment of polymer viscoelasticity, including time-temperature superposition, was established by Ferry and others in the 1950s and 1960s.

Key figures

• Paul Flory
• John Ferry
• Leslie Treloar

Seminal works

• sperling2006
• flory1953

Frequently asked questions

Why does the same polymer feel stiff when struck quickly but flows slowly under sustained load?

Polymers are viscoelastic: at short times or high rates the chains cannot rearrange and respond elastically, while at long times they relax and flow. This time dependence underlies creep, stress relaxation, and rate-sensitive toughness.

Why does a rubber band get stiffer when heated?

Rubber elasticity is entropic. Stretching lowers the conformational entropy of the network chains, and the restoring force is proportional to absolute temperature, so heating increases the retractive force rather than softening the rubber.