Isothermal crystallization kinetics and melting behaviour of poly(ethylene terephthalate)

Differential Scanning Calorimetry has been used to study the isothermal crystallization kinetics and melting behaviour of PET. Kinetic analysis indicated that the overall crystallization of PET involved two processes, attributed to primary and secondary crystallization. Secondary crystallization occurred consecutively with primary and both processes obey different Avrami time dependences. The primary process was that of heterogeneous nucleation and three-dimensional spherical growth that was confirmed by direct observation of spherulites by SEM. Secondary crystallization was that of one dimensional growth involving fibrillar growth between the primary lamellae of the spherulites. Accordingly primary crystallization has a stronger temperature dependence on temperature than secondary. Further analysis based on Hoffman-Lauritzen theory revealed that PET crystallization followed regime I kinetics at temperatures between 490 and 564 K. Below 490 K, regime II kinetics were operational. Multiple endotherms were observed in melting PET and attributed to the effect of crystal perfection and re-crystallization on heating from the crystallization temperature to the m.pt. Increasing the crystallization temperature and the rates of heating during melting scans minimized these effects. Increases in yield stress, yield strain and decrease in elongation at break with crystallinity were ascribed to the strengthening effect of the crystals on the amorphous matrix, accompanied by the change in mechanism of tensile deformation from ductile yielding to craze-crack growth. (C) 2001 Elsevier Science Ltd. All rights reserved.