REVERSIBLE MELTING IN POLYMER CRYSTALS DETECTED BY TEMPERATURE-MODULATED DIFFERENTIAL SCANNING CALORIMETRY

A small amount of locally reversible melting and crystallization in po ly(ethylene terephthalate) (PET) has been detected by temperature-modu lated differential scanning calorimetry (TMDSC). Extended-time TMDSC w as 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 crystallizati on is less than 10% of the total heat of fusion and decreases further with time over many hours. The new observations support the concept th at ''molecular nucleation'' is the reversible and rate-determining ste p in crystallization. On TMDSC, partially-melted chains remain on the surface of higher-melting crystals to permit crystallization during th e cooling cycle without supercooling.