The behaviour of amorphous and crystalline polymers under applied heat is quite different.
In the solid state, a crystalline polymer is rigid with high viscosity. As the polymer is heated to past its melting point the viscosity drops abruptly to that of a flowing liquid. This is similar to ice when it melts into water.
When a crystalline polymer is heated from the solid state it will expand at a constant rate dependent on its coefficient of thermal expansion. At the melting point the polymer undergoes a large increase in volume before once again expanding at a constant rate in the liquid state.
The large expansion, or increase in free volume, occurs as the chains in the crystalline state breakdown into an amorphous structure.
After Engineering Materials: Properties and Selection, K. G. Budinski, 8th Edition, Pearson Education International.