1. Amorphous and crystalline polymers

Several observations can be made:

• When melted, linear polymers are usually highly viscous fluids;

• some of them change very gradually from the solid state to the rubbery and then viscoelastic state (amorphous polymers); others become transparent in the molten zone, remain translucent or opaque in the solid part, and the boundary between the two zones is clearly visible (partially crystalline polymers).

Careful observation of the phenomena that accompany melting shows that each polymer gives rise to a different scenario:

• PMMA, for example, disappears in the heated zone and small bubbles form on the degraded surface;

• Rigid PVC changes color, turning yellow, then brown, then black, and generally...