Elastic Recovery and Viscoelastic Behaviour

Elastic recovery of polyester: >90% at 5% extension (recoverable elastic strain stored in crystalline network); drops to 60–70% at 10% extension (permanent set in amorphous chains). Creep under constant load: J(t) = Je + t^n/η (power-law creep, n~0.25 for PET). Stress relaxation: σ(t) = σ₀·exp(−t/τ). Dynamic mechanical analysis: storage modulus E', loss modulus E'', tan δ peak at Tg (β-relaxation ~120°C, local chain motion; α-relaxation at Tg).