Relationship between Sequence Distribution and Thermal Properties of the Transesterification Product between Poly(ethylene terephthalate) and Poly(butylene terephthalate)

The relationship between the number-average sequence length and the melting temperature of the block copolyesters prepared by quench after the transesterification reaction between poly(ethylene terephthalate) and poly(butylene terephthalate) was reported. The number-average sequence length was determined by the triad sequence analysis in a solvent system of o-chlorophenol/deuterated chloroform mixture (75/25 v/v) at 80 °C using 600 MHz 1H NMR. The melting temperature was observed using a differential scanning calorimeter. The linear correlation between the melting temperature and the inverse of the number-average sequence length was observed for the transesterification products with composition of PET/PBT from 20/80 to 80/20 mol %. This correlation was confirmed by the measurement of the long period using small-angle X-ray scattering. For the transesterification products obtained under definite conditions such as the quench after the transesterification reaction, it was confirmed that the number-average sequence length measured by NMR can be approximately used instead of the lamellae thickness in Gibbs−Thomson equation that has been well-known as the relationship between the melting temperature and the lamellae thickness. The decrease in the ratio of the relative peak area of the methylene carbon of the butylene glycol and ethylene glycol units, I PBT/I PET, in the solid-state 13C NMR spectra was observed as the progress of the transesterification. This seems due to change in the local motion of each segment reflecting the decrease of the crystalline size.