In the molten state, the polymer has large relative molecular weight, high viscosity and poor fluidity. Under isobaric condition, Cp is greatly influenced by its own intermolecular force.

For amorphous polymers, the heat capacity Cp increases almost linearly with temperature in two temperature regions below or above the glass transition temperature Tg, and there is a sudden change at Tg. For the partially crystalline polymer, there is a very obvious peak at the crystallization melting point Tm, because the melting of the crystalline part of the polymer requires a lot of heat.