The melting of poly(ethylene terephthalate) is analyzed by quasi-isoth
ermal, temperature-modulated differential scanning calorimetry. The me
asurement is done by sinusoidally changing the temperature in the melt
ing range (+/-1.0 K). In the melting range a small portion of the samp
le melts reversibly. This observation is taken as a direct observation
of the reversibility of melting of specific macromolecules as long as
they are melting only partially and need no molecular nucleation for
recrystallization.