Triple melting behavior of poly(ethylene terephthalate co-1,4-cyclohexylene dimethylene terephthalate) random copolyesters

The melting behavior of poly(ethylene terephthalate co-1,4-cyclohexylene dimethylene terephthalate) [PET/CT] random copolyesters has been studied. The basis of this analysis was the triple melting behavior of PET homopolymers, which is commonly observed after a period of isothermal crystallization followed by linear heating in a differential scanning calorimeter. Both ET and CT homopolymers are able to crystallize, and as a consequence, the copolymer morphology depends on the ET/CT ratio. It has been reported that at low CT concentrations, the ET units can crystallize with complete rejection of the CT units and that at high CT concentrations, the CT units can cocrystallize with the ET units. In the present work, low CT concentrations were selected, as they are completely rejected from the ET crystals. The purpose was to further test the hypothesis that in the triple melting behavior of PET homopolymers, the second DSC melting endotherm is related to secondary species crystallized by material rejected from the primary crystals. This concept arose from our previous work, where it was speculated that increasing the average molecular-weight of PET would enhance molecular entanglement and increase secondary crystallization. This process would give rise to a higher amount of species being rejected from the main crystals, i.e., an increase of secondary crystallization would occur, and as a consequence the second melting endotherm would be enhanced. Similar to the effect of molecular weight, such behavior has been observed as a function of rejected copolymer content. This gives support to our previously proposed step-like crystallization and melting mechanism for PET homopolymers, and has the potential to be extended to other high temperature semicrystalline polymeric systems. (C) 1998 John Wiley & Sons, Inc.