Isothermal crystallization kinetics and melting behaviour of poly(ethyleneterephthalate)

Differential Scanning Calorimetry has been used to study the isothermal cry stallization kinetics and melting behaviour of PET. Kinetic analysis indica ted that the overall crystallization of PET involved two processes, attribu ted to primary and secondary crystallization. Secondary crystallization occ urred consecutively with primary and both processes obey different Avrami t ime dependences. The primary process was that of heterogeneous nucleation a nd three-dimensional spherical growth that was confirmed by direct observat ion of spherulites by SEM. Secondary crystallization was that of one dimens ional growth involving fibrillar growth between the primary lamellae of the spherulites. Accordingly primary crystallization has a stronger temperatur e dependence on temperature than secondary. Further analysis based on Hoffm an-Lauritzen theory revealed that PET crystallization followed regime I kin etics at temperatures between 490 and 564 K. Below 490 K, regime II kinetic s were operational. Multiple endotherms were observed in melting PET and at tributed to the effect of crystal perfection and re-crystallization on heat ing from the crystallization temperature to the m.pt. Increasing the crysta llization temperature and the rates of heating during melting scans minimiz ed these effects. Increases in yield stress, yield strain and decrease in elongation at break with crystallinity were ascribed to the strengthening effect of the crysta ls on the amorphous matrix, accompanied by the change in mechanism of tensi le deformation from ductile yielding to craze-crack growth. (C) 2001 Elsevi er Science Ltd. All rights reserved.