Rigid amorphous fraction and melting behavior of poly(ethylene terephthalate)

The multiple melting behavior of poly(ethylene terephthalate) (PET) is generally attributed to the fusion of original crystals recrystallized during the heating at conventional scanning rate. In the present study, the triple and double melting behavior that is observed after isothermal crystallization at T_c lower and higher than 215°C, respectively, is put in relation with the presence and absence of rigid amorphous fraction around the original primary crystal lamellae. The complex melting behavior is explained by assuming that two different morphologies of primary crystals develop during crystallization at temperatures lower than 215°C, in a proportion that is a function of the crystallization temperature: chain cluster aggregations with a high percentage of rigid amorphous fraction on the boundaries and small crystals with a high percentage of adjacent reentry folding and reduced constraints at the amorphous/crystal interphase. These distinct morphologies differently transform upon heating at low scanning rate, originating two endotherms. On the contrary, after crystallization at T_c>215°C, all the primary crystalline structure, which probably are characterized by the same morphology made of tightly chain folded lamellae and absence of rigid amorphous fraction, undergo the same reorganization route, originating a single endotherm.