Transesterification Reaction and Morphology of PET/PEN Melt-mixed Blends and Their Flexural Strengths

Poly(ethylene terephthalate) (PET) / poly(ethylene naphthalate) (PEN) blends with 50/50 composition have been prepared by melt-mixing at the temperatures above 280°C and subsequent heating at 280°C before hot-press molding. Progress of transesterification reaction and formation of PET-PEN random copolymers during these melt-blending processes have been confirmed by 1H-NMR measurement. Dynamic viscoelasticity, DSC thermograms, and SEM images for the PET/PEN blends with shorter heating time at 280°C suggested that the blend sample was in an immiscible state when the transesterification level (T) was less than 5%. With the increase of the heating time, the sample changed to miscible state with T > 5 % and exhibited transparency. The phase-separated morphology was fine and the size of the phase-separated domain was several hundred nanometer when T ≈ 5%. Flexural test for the blend samples revealed that the PET/PEN blends exhibited the maximum flexural strength when T ≈ 5 %. Increase in the melt-mixing temperature was found to accelerate the transesterification reaction, and to shorten the processing time to obtain the miscible PET/PEN blend.