Increasing lamellar twisting frequency with poly(lactic acid) segments incorporation in poly(trimethylene terephthalate) ring-banded spherulites

In this study, a series of random poly(trimethylene terephthalate)-co- polylactide copolyesters (PTT-co-PLA) were synthesized by melting copolycondensation, and the influence of PLA segments on the lamellar twisting frequency of PTT was studied. Etching technology was used to study lamellar morphologies of ring-banded spherulites and distinct images of twisting lamellae in crystalline PTT and PTT-co-PLA were observed by scanning electron microscopy. Comparing with PTT homopolymer, ring-banded spherulites in PTT-co-PLA have much smaller band spacing and the band spacing decreases sharply with the increase of PLA segments content. It was confirmed by experiments that there are 2-fold reasons for the sharp decrease of band spacing in PTT-co-PLA ring-banded spherulites. First, the unbalanced surface stress at the fold surfaces for the PTT lamellar twisting can be intensified by amorphous PLA segments. Second, the thickness of PTT lamellae decreases with the increase of PLA segments content. Both changes significantly enhance the helical twisting frequency of twisted lamellae.