Polymers: Chains, Transitions and Time-Dependent Behaviour
A practical introduction to molecular architecture, glass transition, crystallinity, viscoelasticity and polymer processing.
Chains govern behaviour
Polymers consist of long molecular chains whose length, branching, side groups and crosslinking control mobility. Thermoplastics soften repeatedly, thermosets form permanent networks, and elastomers combine flexible chains with light crosslinking.
Glass transition and melting
The glass-transition temperature marks a major change in segmental mobility. Below it, many polymers are stiff and glassy; above it, they become more compliant. Semi-crystalline polymers also possess a melting range associated with ordered regions.
Time and temperature are coupled
Polymer response is viscoelastic: stiffness, deformation and recovery depend on loading time and temperature. A polymer that is rigid during a short test may creep substantially under a sustained load.
Processing creates structure
Extrusion, injection moulding, blow moulding, thermoforming and additive manufacturing orient chains and create cooling-rate gradients. These effects influence shrinkage, residual stress, warpage and anisotropy.
Working rule
Specify service temperature, loading duration, environment and manufacturing history together. Room-temperature tensile strength alone is rarely enough.