Crystal Structure and Tensile Fracture Morphology of Poly(ethylene terephthalate)-co-poly(ethylene 2,6-naphthalate) Block Copolyesters and Fibers

Poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) homopolyesters were synthesized and then copolymerized at different molar feed ratios into PET-co-PEN (TN) block copolyesters. The sequence distribution of the TN copolyesters was analyzed through proton nuclear magnetic response spectroscopy. The results demonstrated that the block segments of PET and PEN are highly miscible and tend to be randomly or nearly randomly distributed in the synthesized TN molecular chains. For TN block copolyesters with PET as the main component and number-averaged sequence length of PET > 3, wide-angle X-ray diffraction (WAXD) profiles indicated that all TN copolyester crystals were related to PET. TN block fibers were produced from a TN block copolyester through melt spinning at a 90:10 PET-to-PEN molar feed ratio and then subjected to different heat treatments. The relaxation of the oriented amorphous chain at elevated temperatures reduced the birefringence of the TN fiber. When the relaxation heat treatment time or stretching was longer, the oriented amorphous molecular chains were reoriented and crystallized, which increased the birefringence of the TN fiber. The differences in the segmental mobility of PET and PEN led to a multinecking and a mushroom-head morphology with transverse striations along the fiber axis because of the snapback effect after tensile failure.