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.
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