A small amount of locally reversible melting and
crystallization in poly(ethylene terephthalate) (PET) has been detected by temperature-modulated differential
scanning calorimetry (TMDSC).
Extended-time TMDSC was used in the quasi-isothermal mode.
Studied were melt-crystallized, quenched,
and a biaxially-oriented film of PET in temperature steps of 2 K from
320 to 560 K. The integral of the
endothermic and exothermic latent heat contributions to the reversible
melting and crystallization is
less than 10% of the total heat of fusion and decreases further with
time over many hours. The new
observations support the concept that “molecular nucleation” is the
reversible and rate-determining step
in crystallization. On TMDSC, partially-melted chains remain on
the surface of higher-melting crystals
to permit crystallization during the cooling cycle without
supercooling.
The melting of poly(ethy1ene terephthalate) is analyzed by quasi-isothermal, temperature-modulated differential scanning calorimetry. The measurement is done by sinusoidally changing the temperature in the melting range (*1.0 K). In the melting range a small portion of the sample 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.
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